Agent Communication

1
Agent Communication

• Sensor Web TITAN Research
• 12/22/2009

2
Agent Communication

• A fundamental charactistic of MAS
• Individual agents communicate and interact
• Accomplished through the exchange of messages
• To understand each other, agents agree on the
  format and semantics of these messages.
• Messages adhere strictly to the ACL (Agent
  Communication Language) standard which allows
  several possibilities for the encoding of the
  actual content

3
Structure of a JADE message

• Performative FIPA message type
• Addressing
• Receiver
• Sender Initialized automatically
• Content Message content
• ConversationID Links messages in the same
  conversation

4
Structure of a JADE message (cont.)

• Language
• Ontology
• Protocol
• ReplyWith
• InReplyTo
• ReplyBy

5
Creating a message

• To create a message, you have to indicate its
  type (performative) and set the content.
• ACLMessage msg new ACLMessage(
  ACLMessage.INFORM )
• msg.setContent("I sell seashells at 10/kg" )

6
ACL Performatives

• INFORM One agent gives another some useful
  information
• QUERY One agent asks another a question
• REQUEST One agent asks another to do something
• PROPOSE Used to start bargaining between agents
• AGREE Accept a proposition
• REFUSE Refuse a proposition

7
Sending messages

• With an Agent ID (AID) for the recipient, sending
  a message is fairly easy
• AID dest ...
• msg.addReceiver( dest )
• send(msg)
• Note addReceiver allows for adding several
  receivers to the message and the one send
  broadcasts it to all of them.

8
Receiving messages

• Messages are routed to the receiving agents
  message queue.
• 2 basic ways for the receiver to get its
  messages.
• blockingReceive() agent suspends all its
  activities until a message arrives
• ACLMessage msg blockingReceive()
• receive() examines the message queue, returning
  a message if there is one or null otherwise. This
  is the normal technique used when an agent is
  involved in parallel activities and has multiple
  active Behaviours.
• ACLMessage msg receive()
• if (msg ! null)
• lt.... handle message...gt
• else
• lt... do something else like block() ...gt
• These methods have variants with additional
  arguments to alter their behaviour. With
  blockingReceive(), one can add a pattern object
  to select the kinds of message that we want to
  receive.

9
Receiver Example

• public class Receiver extends Agent
• protected void setup()
• addBehaviour(new CyclicBehaviour(this)
• public void action()
• ACLMessage msg receive()
• if (msg!null)
• System.out.println( " - "
• myAgent.getLocalName() " lt-
  "
• msg.getContent() )
• block()
• )

10
Receiving messages w/ block()

• NOTE block doesn't stop execution, it just
  schedules the next execution
• Without block, the behaviour is essentially a LOOP

11
Answering messages

• All messages have a sender ID attribute, so we
  can answer a message as follows
• ACLMessage msg receive()
• ACLMessage reply new ACLMessage(ACLMessage.
  INFORM)
• reply.setContent( "Pong" )
• reply.addReceiver( msg.getSender() )
• send(reply)

12
Answering messages

• public void action()
• ACLMessage msg receive()
• if (msg!null)
• System.out.println( " - "
• myAgent.getLocalName() "
  lt- "
• msg.getContent() )
• ACLMessage reply
  msg.createReply()
• reply.setPerformative(ACLMessa
  ge.INFORM)
• reply.setContent(" Pong" )
• reply.send()
• block()
• NOTE The advantage to using createReply appears
  when other attributes like conversationID or
  ontology have to correspond to the original
  message.

13
Searching the AMS for agents to talk to

• The AMS keeps track of all agents in the system
• Searching the AMS an array of all presently known
  agents

14
Searching the AMS Example

• // Required imports
• import jade.domain.AMSService
• import jade.domain.FIPAAgentManagement.
• ...
• AMSAgentDescription agents null
• try
• SearchConstraints c new
  SearchConstraints()
• c.setMaxResults ( new Long(-1) )
• agents AMSService.search( this, new
  AMSAgentDescription (), c )
• catch (Exception e) ...

15
Ping Pong Example

• Sends Ping messages to all agents, including
  itself and the 2 system agents
• Includes a search of the AMS for the agents

16
Ping Pong Example

• AMSAgentDescription agents null
• try
• SearchConstraints c new SearchConstraints()
  c.setMaxResults (new Long(-1))
• agents AMSService.search( this, new
  AMSAgentDescription (), c )
• catch (Exception e)
• System.out.println( "Problem searching AMS " e
  ) e.printStackTrace()

17
Ping Pong Example (cont.)

• ACLMessage msg new ACLMessage(ACLMessage.INFORM)
  msg.setContent( "Ping" )
• for (int i0 iltagents.lengthi)
• msg.addReceiver( agentsi.getName() )
• send(msg)
• addBehaviour(new CyclicBehaviour(this)
• public void action()
• ACLMessage msg receive()
• if (msg!null)
• System.out.println( " Answer" " lt- "
  msg.getContent() " from " msg.getSender().getN
  ame() )
• block()
• )